1. Field of the Invention
The present invention relates to a radiation detector with at least two sensing elements and more specifically to a detector having a plurality of radiation sensitive fields of view with an asymmetric pattern.
2. Description of the Prior Art
It has been known to use radiation detectors sensitive to infrared radiations to detect persons moving in a room. The infrared detectors are sensitive to the normal IR radiation from the human body. These types of detectors are commonly used as intruder alarms and the like. It also has been common to concentrate the sensitivity of the radiation detector into a plurality of very narrow solid angles, called fields of view, so as to cover a desired area such that a person entering that area will pass through at least one of such fields of view.
U.S. Pat. No. 2,198,725 teaches apparatus that produces a number of stripe and beam shaped fields of view by means of a grid pattern in front of the radiation detector. Such beam shaped fields of view can also be produced by an arrangement of concave mirrors adjusted to point in the appropriate direction such as disclosed in U.S. Pat. Nos. 3,703,718 and 4,058,726. Through the use of lenses, such as Fresnel lenses and objectives, each beam can be adjusted to point in a desired angle with each lens or objective producing one field of view. Swiss Pat. No. 596,619 describes a system using various combinations of plain, cylindrical, and double curved mirrors and lenses. These prior art devices and systems are intended to cover a plurality of discrete fields of view simultaneously using either a single radiation detector, or an array of radiation detectors mounted adjacent to each other.
Early intruder alarms or motion detectors have used a single radiation detector having sufficient sensitivity to detect people or other objects whose temperature differed from the ambient temperature. Although these detectors were satisfactory during conditions of uniform ambient temperature, the detectors were very sensitive to disturbances in the ambient temperature such as turbulences of air due to heating and air conditioning systems, to sunlight, external vehicle headlights, and other sources of radiation which would lie in the spectral sensitivity range of the sensor. The basic problem with the early systems was that numbers of false alarms were created, decreasing the effectiveness of their use in security systems.
The high false alarm rate problem was solved subsequently using at least two closely spaced sensing elements connected to form a differential sensor. For example, in a dual element system, one element produces a positive going signal and the other element produces a negative going signal when exposed to the same radiation. Thus, if the ambient temperature changes or an external source such as air turbulence or sunlight strikes the unit, each element will be equally energized and the two signals will cancel. Thus, signals from such ambient sources are essentially rejected when the system is symmetrical and exactly balanced. However, the two sensors will have different fields of view or sensitivity patterns. A person moving through the fields of view will normally be detected first by one element and later by the other element. Thus, no cancellation takes place and both signals are passed to suitable amplifiers and relays to produce a desired alarm or other indication.
A major disadvantage of the above described multielement systems is that they are not able to detect moving objects very close to the sensor since the fields of view of the multiple sensors will necessarily overlap in that range. It has been found that an intruder moving close to this type of sensor used in a security system will not be detected and can tamper with or cover the sensing elements without detection.
While attempts have been made to solve this problem by combining a single sensor and a differential sensor, producing asymmetry in a differential sensor, and similar approaches, the result has been a system which immunity to disturbances is reduced and represents a compromise. Thus, there is a need for an infrared motion or body detecting system which will be free from false alarms due to ambient disturbances but that will detect objects or bodies at both near and far ranges.